An exhaust system of a motor vehicle may include one or more sensors responsive to exhaust constituents: nitrogen oxides (NOx), ammonia, carbon dioxide, particulates, water vapor, and/or oxygen, for example. Coupled in an exhaust conduit of the exhaust system, the sensors may be used to diagnose the efficacy of emissions control, and in some cases, to modify emissions-control or engine-system parameters to improve performance. For example, the output of an ammonia sensor coupled downstream of a selective catalytic reduction (SCR) reactor may be used to regulate the amount of ammonia supplied to the SCR reactor.
To conserve space, limit costs, and accelerate warm-up, motor-vehicle exhaust sensors are typically compact, low-profile devices. This means that a sensor may sample only a relatively small portion of the exhaust flow in the conduit in which it is coupled. However, the various exhaust constituents flowing through the conduit may be distributed inhomogeneously. Accordingly, some form of homogenization or multiple-sampling may be desired so that the output of the sensor accurately reflects the area-averaged concentration of the sensed constituent.
One approach is to conduct a portion of the exhaust flow through an exhaust-gas sampling tube—e.g., a hollow tube extending radially into the exhaust conduit. The tube may include a series of inlets arranged normal to, and facing, the exhaust-flow direction in the conduit. The inlets may receive and sample the exhaust flow at several positions along the conduit radius. The sampled exhaust then travels up the tube, across the sensor element, and is discharged through an outlet hole, also normal to the flow direction, on the opposite, downstream side of the tube. However, this configuration still may not provide adequate sampling of inhomogeneously distributed constituents in an exhaust flow.
Therefore, one embodiment of this disclosure provides an improved method for sensing an engine-exhaust constituent. The method comprises separating a first exhaust flow from a second exhaust flow by admitting the first exhaust flow into an envelope. The method further comprises guiding the first exhaust flow across a sensor coupled in the envelope, and guiding the second exhaust flow over an exterior surface of the envelope to induce a pressure minimum. The method further comprises releasing the first exhaust flow from the envelope through an outlet arranged at the pressure minimum. Another embodiment provides an exhaust-gas sampler adapted to couple into an exhaust conduit and to sample a constituent of an engine exhaust flowing therein. The sampler comprises an envelope having upstream and downstream surfaces joined by a curved side surface. The upstream and downstream surfaces are each tangent to a plane substantially normal to a central axis of the exhaust conduit. A series of inlets are formed in the upstream surface, and an outlet is formed in the curved side surface. A sensor responsive to a level of the constituent in the exhaust may be coupled in the envelope, at the peripheral region of the conduit. This sensor, by virtue of the exhaust-gas sampler and related methods disclosed herein, generates an output less affected by inhomogeneous pockets of exhaust constituents and more representative of the overall composition of exhaust gas flowing through the exhaust conduit.
The summary above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this summary nor to implementations that address problems or disadvantages noted herein.